1 . Field of the Invention
The present invention relates generally to improvements in a cellular mobile communications system, and more specifically to a method and arrangement of effectively reducing the number of electric field strength measurements which are required for a handoff in such a communications system. The present invention finds extensive use in the case where a service zone (viz., cell) is sub-divided into smaller ones in order to meet increase in the number of cellular subscribers.
2 . Description of Related Art
It is well known in the art to apply frequency reuse in a mobile communications system in order to satisfy various objectives such as large subscriber capacity, efficient spectrum use, widespread availability, adaptability to traffic density, etc.
Frequency reuse refers to the use of the same radio carrier frequency in a number of different channels to cover different areas which are separated from one another by distances sufficient that objectionable cochannel interference does not occur. The so called cellular mobile communications system utilizes such a concept.
When a mobile unit, during a call, moves out of the service coverage of a particular land site, the signal strength measured at the particular land site falls below a handoff threshold level. Thus, the present land site requests a MTSO (Mobile Telephone Switching Office) to implement a handoff or switch over the mobile unit to another suitable nearby land site. The MTSO, in response to the handoff request, instructs the neighboring land sites to measure the strength of the signal transmitted from the mobile unit to be handed off.
After each of the nearby land sites completes measuring the signal strength of the mobile unit to be handed off, the land site informs the MTSO of the results. The MTSO checks the results which are received and then implements a handoff if there is a land site to which the mobile unit can be suitably handed off.
On the other hand, in order to meet the ever increasing number of cellular subscribers, it is a current practice to subdivide each cell into smaller ones (viz., so-called micro-cells).
This cell subdividing, however, inevitably increases the total number of handoff requests. In addition to this, upon a handoff request being generated from a given land site, the MTSO has to instruct the signal strength measurement for the handoff to the land sites the number of which is relatively large as compared with the case where the cell is not subdivided. In more specific terms, the MTSO is required to implement a handoff without failure even though a mobile unit moves at a high speed from cell to cell without cutting off communication. Accordingly, the MTSO instructs a relatively large number of land sites which includes, in addition to the land sites surrounding the land site which issued a handoff request, another land sites which are located outside the surrounding ones.
The increase in the number of the signal strength measurements for a handoff, undesirably renders complex the overall system arrangement because of the increase in the number of traffics for measuring the signal strength for the handoff.
It is therefore highly desirable to reduce the number of signal strength measurements in the case where a mobile unit moves at a low speed.